A molecular formula shows the actual number of atoms of each element in one molecule. The empirical formula shows the simplest whole-number ratio. To get the empirical formula from a molecular formula, divide every number by the largest number that goes into all of them. Glucose, C6H12O6, divides by 6 to give CH2O, so the ratio of C : H : O is 1 : 2 : 1. Ethane, C2H6, divides by 2 to give CH3. Some formulae, such as H2O and CH4, are already as simple as they can be.
To go the other way you need the relative formula mass of the molecule. Work out the empirical formula mass by adding up the relative atomic masses. Divide the relative formula mass of the molecule by the empirical formula mass to get a whole number, n. Then multiply every number in the empirical formula by n.
Example: the empirical formula is CH2 and the molecule has a relative formula mass of 56. The empirical formula mass is 12 + 2 = 14, so n = 56 ÷ 14 = 4 and the molecular formula is C4H8. Example: for CH with a relative formula mass of 78, the empirical formula mass is 13, so n = 78 ÷ 13 = 6 and the molecule is C6H6. Check with ethane: CH3 has a mass of 15 and ethane has a relative formula mass of 30, so n = 30 ÷ 15 = 2.